This kind of packaging bag is made of polyethylene.
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Break it down: 聚 (poly-) + 乙烯 (ethylene) = polyethylene. 乙 is the second Heavenly Stem, used in chemistry to mean "ethyl" (2-carbon), and 烯 means "alkene." Many polymers follow this 聚+monomer pattern: 聚氯乙烯 (PVC), 聚丙烯 (polypropylene).
